1. Field of the Invention
The present invention relates to an aqueous hydrophilization treatment composition and method for an aluminum-containing metal material. More particularly the present invention relates to an aqueous hydrophilization treatment composition and method for an aluminum-containing metal material usable in heat exchangers, for example, evaporators for car air-conditioners and in parts for the heat exchangers.
2. Description of the Related Art
It is known that almost all of the conventional heat exchangers made from an aluminum-containing metal material are designed so that the surface areas of heat-dissipation surface portions and heat absorption (cooling) surface portions (fins) are made as large as possible to enhance the heat dissipation effect and thus the cooling effect of the heat exchangers.
Also, it is known that to make the size of heat exchangers as small and compact as possible, the gaps between the fins are made very small. Accordingly, when the conventional heat exchangers work, moisture in the ambient air is condensed in the fin gaps. In this moisture condensation phenomenon, the higher the degree of hydrophobic property of the fin surfaces, the easier the formation of water drops from the condensed water on the fin surfaces, and thus the fin gaps are blocked by the water drops. The blockade of the fin gaps results in an increase in resistance to air flow and thus in a decrease in heat-exchange efficiency. Also, the water drops formed between the fin gaps are scattered into the downstream side of the heat exchange so as to reduce the heat exchange efficiency.
In almost all of the conventional heat exchangers, the heat exchanging surfaces are chemical conversion-treated with, for example, a chromate treatment, to enhance the resistance to corrosion. The conventional chemical conversion coatings have a low hydrophilicity and thus cannot solve the above-mentioned problems. Also, the conventional chemical conversion coatings are disadvantageous in that they generate an offensive odor.
Further, the conventional heat exchangers have a problem in that microorganism derived from the condensed water and from the dust in the air adhere to the heat exchangers are propagated in the gaps between the fins, and the propagated microorganisms produce metabolic products which generate a putrid odor.
As a method of hydrophilizing the fin surfaces, Japanese Examined Patent Publication (Kokoku) No. 53-48177 discloses a chemical conversion process characterized by treating an aluminum material with an aqueous alkali silicate solution having a specific composition and then heat-drying the treated surface at a temperature of 150.degree. C. or more. Also, Japanese Examined Patent Publication (Kokoku) No. 57-46000 discloses a surface-treatment method characterized in that aluminum-made heat exchanger surfaces are treated with a solution containing porous fine particles and an organic polymeric resin. Further, Japanese Unexamined Patent Publication (Kokai) No. 60-101156 discloses a coating-forming agent for aluminum, characterized by comprising an alkali silicate, a low molecular organic compound having a carbonyl group and a water-soluble organic polymeric compound. In these prior arts, an application of an inorganic hydrophilic compound, for example, a silica sol or an alkali silicate compound to the aluminum material is disclosed.
However, in a pre-coat system in which a coating treatment is applied to a metal material and then the coated metal material is subjected to a forming process, there is a problem that where an aluminum article surface treated with a treatment agent containing the above-mentioned inorganic compound having a high hardness is subjected to an ironing process or perforating process, the processing tools and molds are worn due to high abrasion. Also, the hard coating has no preventive effect on the generation of the offensive odor and on the contrary, promotes the generation of the offensive odor.
To solve the above-mentioned problems, some processes, in which an organic compound-containing treatment liquid free from the inorganic compound, such as the alkali silicate, is used, are provided.
For example, Japanese Unexamined Patent Publication No. 60-219285 discloses an anti-fogging and anti-water drop treatment agent comprising, as principal components, a polyamide having a tert-amine group and/or a polyamide having a polyalkylene glycol group. When this treatment agent is applied, however, the resultant treated metal material exhibits an unsatisfactory durability in hydrophilicity during a long period of use, and the problem that an offensive order, for example, a putrid odor, is generated during a long period of use, is not solved.
As means for preventing the scattering of the condensed water drops and the generation of the offensive odor, Japanese Unexamined Patent Publication No. 61-250495 discloses a process for forming a hydrophilic coating comprising, as a principal component, a water-soluble polyamide resin which exhibits a cationic property in an aqueous solution, on a chemical conversion coating, for example, a chromate coating, formed on a metal material. Also, Japanese Unexamined Patent Publication (Kokai) No. 3-72562, discloses a surface hydrophilization treatment liquid comprising a partially saponified polyvinyl acetate, a water-soluble nylon and a water-soluble amino resin. The above-mentioned treatment process and agent are not always satisfactory in durability in the hydrophilization effect and the offensive-odor-prevention effect.
Japanese Unexamined Patent Publication (Kokai) No. 2-219,875 discloses a hydrophilization coating agent characterized by being produced by copolymerization of four types of .alpha. and .beta.-unsaturated monomers having specific functional groups. This hydrophilization coating agent can reduce the wear of molds used in the forming process. In this publication, it is stated that a polyepoxide which is a glycidyl compound, is used as a cross-linking agent for the copolymer. However, it is difficult to produce the target copolymer from the four monomers having ionic functional groups different in ionic property from each other, and the resultant copolymer is chemically unstable. Therefore, there is a problem in the pot-life of the resultant hydrophilization coating liquid. Also, the resultant coating produced by the treatment of the Japanese publication exhibits substantially no effect on the prevention of the offensive odor generation.